1. Field of the Invention
The present invention relates to a programmable logic controller (hereinafter abbreviated as “PLC”), and to a programming tool for editing a sequence program for the PLC.
2. Description of the Related Art
A distributed control system composed of a plurality of PLCs that coordinate with (cooperate) each other to perform sequence control is occasionally used in a control device for production facilities or the like (see Japanese Patent Application Publication No. 2008-262453 (JP 2008-262453 A)). A program corresponding to a sequence circuit generated in units of modules and an interface circuit for communication among the PLCs, for example, is written into each of the PLCs. In the case where a data link scheme is adopted, the PLCs execute sequence control with data for communication via a network held in an allocated prescribed memory region.
In the distributed control system described above, the number of PLCs is occasionally increased and decreased as the distributed control system is functionally expanded, partially functionally disabled, or the like to change the configuration of the distributed control system. In such a case, it is necessary to correct the program corresponding to the sequence circuit or the interface circuit or the like or reallocate the memory region such that sequence control and communication among the PLCs are performed normally in the changed distributed control system. Therefore, a change in configuration of the distributed control system along with increase and decrease in number of PLCs has required additional work such as preparation and correction of a sequence program and writing the program into the PLCs.